In more detail
BMR is the energy for breathing, circulation, temperature control and cell repair, and for most sedentary people it is the largest share of daily energy use, typically around 60 to 70%. The Mifflin-St Jeor equation is 10 × kg + 6.25 × cm − 5 × age + 5 for men and − 161 instead of + 5 for women. The Katch-McArdle equation, 370 + 21.6 × lean mass (kg), uses lean body mass instead and suits people who know their body fat. BMR is multiplied by an activity factor to get TDEE. See How Calorie Needs Are Calculated.
Why it matters
Every calorie target starts from BMR, so an error here carries into everything built on it. Equations are population averages: for most people Mifflin-St Jeor lands within about 10% of a measured value, but individuals can differ more. BMR falls with age and with loss of muscle, one reason energy needs drop over the years. Very low calorie diets should be supervised by a health professional.
Example
Man, 30, 80 kg, 180 cm: 10×80 + 6.25×180 − 5×30 + 5 = 1,780 kcal/day
Woman, 30, 60 kg, 165 cm: 10×60 + 6.25×165 − 5×30 − 161 = 1,320 kcal/day
Katch-McArdle, 80 kg at 15 % fat (68 kg lean): 370 + 21.6×68 = 1,839 kcal/day
Common confusion
BMR is energy at complete rest; TDEE adds movement, exercise and digestion. Resting metabolic rate (RMR) is measured under less strict conditions and comes out slightly higher, but the two are often used interchangeably.